I am analyzing rainfall data in a small region (in a Southeast Asian country) for both past observation data and climate model results for future change.

The main question is that there is a large decreasing trend in the past observation data, annually and seasonally (the trends are between 9% and 25% decrease in 50 years). However, the climate models show increasing trend annually and seasonally with all carbon dioxide emission scenarios (B1, B2, A2).

So if the trends from the climate model with all B1, B2 and A2 scenarios show completely opposite results to the trends from past observation data, then can we say that it might possibly be the fault of the climate model? (If we assume the observation data is good quality).

Climate models are still in their infancy, and will be reworked many times before they can more accurately mimic/predict nature.
IF the observational data are accurate, then we have another bit of field data with which to adjust the models.

I am no great fan of current climate models, in no small part because of the defensive posturing of those who believed the models were accurate.
Perhaps, it was that religious zeal (denier=heretic) for the ultimate truth of obviously flawed models that Lovelock began to refer to the global warming claims/posturing to have become a religion.

I'd go with the field data---how many different sources? Only one = iffy, 2 in agreement = maybe, etc...

The future climate projections are mainly global, not necessarily for the very small (comparatively speaking) area you're focused on. The next IPCC report should give more regional details but that will probably be too late for your purposes.

I presume you're talking about the emissions and climate scenarios from the last IPCC report. The general projections are for both increased precipitation and increased drought. Increased rainfall's not much use if it washes your soil or your crops away rather than providing a useful water supply.

Do you have a particular graph or set of projections you want to reconcile with your own observations? That would be a good basis for discussion if you have it/them.

"Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill"nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke

IF the observational data are accurate, then we have another bit of field data with which to adjust the models.

Climate models are not based on observations. They're based on physics. They are not "adjusted" to fit with observations. That process applies only to meteorology and weather prediction models.

When observations depart from climate modelling - Arctic sea ice being the prime example here - the modellers have to go right back to first principles and change the whole section of the relevant modules. (In the Arctic example they need to change both the ocean heating and heat flow equations/mechanisms and the interface between the ocean and atmospheric modules.)

"Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill"nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke

It's one thing to base the models on physics - it's another thing to get the way various features interact straight. And then there's the ever present problem of computing power.

It's only recently that there's been a computing set-up capable of running a whole earth system all at once. A good many of the modelling problems are about writing runnable programs that will show particular aspects of climate rather than trying to represent the whole of the system. And there is no single whole-of-the-system program at all, the ones they've got take too long to run if they go for too much detail as it is now. It's good to have a suite of programs that you can assemble for particular purposes - the set up is very different when you want to run something for 200 years as against 200000 years.

"Courage is what it takes to stand up and speak; courage is also what it takes to sit down and listen." Winston Churchill"nature is like a game of Jenga; you never know which brick you pull out will cause the whole stack to collapse" Lucy Cooke